Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water

Abstract

The adsorption of ions to aqueous interfaces is a phenomenon that profoundly influences vital processes in many areas of science, including biology, atmospheric chemistry, electrical energy storage, and water process engineering. Although classical electrostatics theory predicts that ions are repelled from water/hydrophobe (e.g., air/water) interfaces, both computer simulations and experiments have shown that chaotropic ions actually exhibit enhanced concentrations at the air/water interface. Although mechanistic pictures have been developed to explain this counterintuitive observation, their general applicability, particularly in the presence of material substrates, remains unclear. Here we investigate ion adsorption to the model interface formed by water and graphene. Deep UV second harmonic generation measurements of the SCN − ion, a prototypical chaotrope, determined a free energy of adsorption within error of that for air/water. Unlike for the air/water interface, wherein repartitioning of the solvent energy drives ion adsorption, our computer simulations reveal that direct ion/graphene interactions dominate the favorable enthalpy change. Moreover, the graphene sheets dampen capillary waves such that rotational anisotropy of the solute, if present, is the dominant entropy contribution, in contrast to the air/water interface.specific ion effects | graphene | SHG spectroscopy | molecular dynamics | adsorption O ver a decade ago, detailed simulations predicted that certain simple ions would exhibit enhanced concentrations at the air/water interface (1), reinvigorating a century-old debate primarily addressing the origin of the famous Hofmeister effects in protein chemistry (2). Experiments subsequently verified these predictions (3, 4). Surface enhancement of simple ions has since been widely studied [e.g., see the review by Jungwirth and Tobias (5)] and attributed to various properties of the ions including size, polarizability, dispersion forces, hydration free energy (6-8), and the degree of interfacial roughness (9-11). Previous temperaturedependent deep UV (DUV) second harmonic generation (SHG) experiments from the Saykally group have determined the enthalpic and entropic contributions to the adsorption of the prototypical pseudohalide, the thiocyanate (SCN − ) ion, showing that a negative enthalpy change drives the ion adsorption, whereas a negative entropy change impedes it (9). Using molecular simulations performed in the same study, the following underlying mechanism was proposed: first, when the ion moves from the bulk to the interface, weakly interacting water molecules are displaced from both the surface and the ion solvent shell into the bulk solution, where they form stronger water-water bonds, leading to the negative enthalpy change. Second, the presence of an ion at the interface dampens its capillary wave fluctuations, leading to the negative entropy change. Although a consensus has yet to be reached regarding the complete theory of interfacial ion adsorption, these collective, detailed studie...

Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface

“…12 It is known that deviations from ideal solvent behavior are related to specific ion effects. [13][14] Although recent experimental, 11,[15][16][17] computational, 18 and theoretical [19][20] advances have improved our understanding considerably, the emerging view is that specific ion effects may not have a single unified explanation, especially when ion-ion interactions and interfacial ion-binding sites are operative. 17 The interfacial interactions of biologically relevant, and usually lighter, ions in aqueous environments have been extensively studied.…”

Charge-Transfer-to-Solvent Spectrum of Thiocyanate at the Air/Water Interface Measured by Broadband Deep Ultraviolet Electronic Sum Frequency Generation Spectroscopy

“…4 Of particular interest is the behavior of ions at interfaces ranging from air/water and polymer/water to metal/water. Recent advances in experiment and theory have revealed mechanistic details of the adsorption of the prototypical thiocyanate ion to air/water, 5,6 dodecanol/water, 7 and graphene/water 8 interfaces, focusing on thermodynamic properties (viz. adsorption free energy, enthalpy, and entropy).…”

Graphene surface structure in aqueous media: Evidence for an air-bubble layer and ion adsorption

“…This again is consistent with the assumption of ion adsorption on graphene. It has been shown by molecular dynamics simulations that graphene could attract, due to π-conjunction and polarizability, both cations [59][60][61] and anions [62,63], such as Na + (ρ = 86.75×10 -6 Å -2 ) and K + (ρ = 55.06×10 -6 Å -2 ) in PBS to the graphene-water interface. Furthermore, the fitted SLD profiles to the XRR curves of the soaked graphene in PBS at 25 °C ( Figure 5c) and 60 °C ( Figure 5d) also show that ion adsorption was promoted upon heating.…”